Pulsation dampening apparatus



y 1960 J. M. SHARP ETAL 2,936,041

PULSATION DAMPENING APPARATUS Filed June 10 1955 2 Sheets-Sheet 1 May1960 J..M. SHARP ETAL I 2,936,041

PULSATION DAMPENING APPARATUS Filed June 10, 1955 2 Sheets-Sheet 2 42(/Q/WGJ A4. J/nrrp Ml/GJ T Hana/ref? dame; f. l/nco/fl Henry W. J/m cwanINVENTORJ United States Paten PULSATION DAMPENINGAPPARATUS James M.Sharp, Miles T. Hanchett, James P. Lincoln, and Henry W. Simpson, SanAntonio, Tex., assignors to Southern Gas Association, Dallas, Tex., acorporation of Texas Application June 10, 1955, Serial No. 514,574 18Claims. Cl. 181-47) This invention relates to improved acoustic filtersof the general type employed to dampen pulsations in a flowing fluidstream such as those created by a compressor, a blower, internalcombustion engine, and the like. In oneof its aspects, it relates 'tosuch a filter in which the transmission of resonant frequencies betweenthe filter components is reduced. In still another of its aspects, itrelates to such a filter in which certain modes of resonant vibrationare discouraged or dampened.

Acoustic filters are frequently employed for dampening pulsations in apulsating gas stream, such as those created by a compressor, internalcombustion engine, blower, and the like. and generally comprise acombinafraction of that of the fundamental.

2,936,041 Patented May 10, 1960 discharge piping. Therefore, in additionto certain fundamental frequencies generated by the compressor, there isalso generated all harmonics of the fundamental "com pressor frequencyor frequencies, albeit that the ampli: tude of the higher harmonics(e.g. 10th) may be only a Nevertheless, some of the higher harmonicspresent as a part of the composite pressure wave generated by thecompressor may frequently have pressure amplitudes ofsutiicientmagnitude as; to excite standing waves of very large pressure amplitildein any piping having a resonance frequency equal or nearly equal to thatof such higher harmonics.

' It' has been found that though acoustic filters can be designed byprocedures theretofore known to have a cutoff frequency below thelcwest,fundamental frequency generated by a compressor orthe like and toeffectively dampen pressure waves of many different frequencies, thefrequency response curve of such filters exhibits pass bands offrequencies having excessively large amplitudes.

As a matter of'fact, the filter often tends to amplify the magnitude ofcertain frequenciesso that, as to such frequencies, the filter is ahinderance rather than a help in decreasing vibration in piping systems.

It has been found that the frequencies passed with or withoutmagnification by the filter, are those with which one or more of thefilter components are in resonance.

, As indicated above, a compressor generates practically tion ofacoustical capacitances and inductances arranged in a filter network.The acousticalcapacitances can be formed from large diameter pipes ortanks (frequently termed bottles) which provide storage ,volume orcapacity within the filter. The acoustical inductances can be in theform of smaller diameter pipes or conduits (frequently termed chokes orchoke tubes) which provide inertia or choking action in the filter. Thefilter elements may be so connected among themselves and with thepumping device that the fluid pumped will pass throot'igh the filter sothat its elements contain pulsating flow of fluid in addition to thesteady flow of fluid. In other words, the filter elements are connectedin series with the pumping device. They may also be connected amongthemselves and with the pumping device in such a manner that the filteris in effect a side branch of the flow line so that it is subject onlyto simple pulsating flow. This may be spoken of as being connected inshunt when speaking in terms of the analogous electrical components. Inany event, flow into or through the filter is the result of'the pumpingaction ofithe device; that is, fluid is intermittently taken from onefilter (the suction filter) and delivered intermittently to anotherfilter (the discharge filter) so that the flow of fluid occurs in moreor less discrete pulses. imposed upon the steady flow of gas, anacoustic pressure wave having an amplitude, frequency and wave-formdependent primarily upon the nature of the pumping device. In manyinstances, this pressure wave is of complex wave-form. Thus, for apiston type compressor, the fundamental frequency at which the suctionand discharge piping is excited by the action of the piston in pumpinggas depends upon the angular velocity of the compressor crank shaft andupon the number of and the crankangle spacing of the'compressor pistons.In multicylinder compressors where the crank angleintervals between theopening of the several suction or discharge valves is not uniform amongthe cylinders, more than one such fundamental frequency can be generatedby the compressor. Further, the action of valve opening and valveclosing, the non-linearity of the gas, the non-sinusoidal motion of thecompressor pitsons, the opposing curvature of the compressionandexpansion pressure-volume curves, etc., all act to distort the shape ofthe pressure wave or waves generated by the compressor in the suctionand all harmonics of its fundamental frequency so that it is almostimpossible to size the filter components to be non-resonant with ,allfrequencies in the composite compressor output pressure wave. Moreoverdue to, the

fact that compressors usually have a range of operating,

speeds, it is even impractical or impossible to sizeithe components fornon-resonance with the lower harmonics which have sufiicient amplitudeto excite sizeable standing Waves. However, it is desirable to eliminateas many as possible of the pass band frequencies from the frequencyresponse curve of the filter, even though its-elements may be inresonance with some of the input frequencies thereto, and such is ageneral object of this invention. 3

Another general object is to provide an acoustic filter which can notonly have a desired low cut-off frequency (ie. be a low pass bandfilter) but which will not pass bands of higher frequencies of excessiveamplitudeftoitsoutput. p t i It is" another general object of thisinvention to-provide an acoustic filter through which there is aminimumof transmission .of pulsing sound energy even though the filter be fedwith pressure waves of complex harmonic structure. 7

Another object is to provide an acoustic filter for dampening pulsationsin fluid systems wherein an arrangement of an acoustical capacitance andinductance is such that transmission of frequencies with which one, ormore of the filter components are in resonance is minimized.

Another object is to provide an acoustic filter in which thetransmission of resonant frequencies between the filtercomponents isminimized by arranging the components to have how junotures with eachother at nodal points of any standing wave tending to exist in thecomponents thereby reducing or eliminating from. the filter frequencyresponse curve, the resonant peaks corresponding to the resonantfrequencies of the filter elements.

Another object is to provide an acoustic filter including an acousticalcapacitance and inductance means interconnected so that a flow juncturewith an acoustical capacitance means is at a distance from the end ofsuch capacitance means, where L is the 24 length thereof and n is aninteger, whereby either the transmission from one filter element toanother of sound energy from standing waves having nodes at suchjuncture is reduced or substantially eliminated or such standing wavesare suppressed by interference.

Another object is to provide such a filter in which the variousacoustical capacitance and inductance units can be arranged so that aselected one, or a combination of more than one, of the units will besubjected to the major portion of the mechanical vibration resultingfrom the filtering action while other filter units and associated pipingare isolated from any substantial mechanical vibration.

Another object is to provide an acoustic filter of the series typewherein the fiow inlet to one or more of the acoustic capacitances is ata node of a standing wave tending to be existent in such capacitancewhereby the magnitude of such standing wave is decreased by destructiveinterference with an incoming wave.

Another object of the invention is to provide a band elimination filterof the resonator type having a maximum efficiency in eliminatingselected frequencies.

Another object is to provide such a band elimination filter in which thefilter elements are connected to the steady flow line at a point suchthat an optimum amount of pulsing energy at the frequency to beeliminated is alternately stored in and released by the filter.

Other objects, advantages and features of this invention will beapparent to one skilled in the art upon a consideration ofthe writtenspecification, the appended claims and the attached drawings wherein:

Fig. 1 is a schematic illustration of a compressor having acousticfilters arranged in its suction and discharge piping system inaccordance with this invention;

Figs. 2 and 3 are diagrammatic illustrations of acoustic filter elementsarranged to illustrate certain principles of this invention; and I Fig.4 is a view showing an acoustic resonator or band elimination filterconnected to a flow line.

In accordance with this invention, standing waves in an acousticalcapacitance means of a filter are suppressed by locating the flow inletto the capacitance means at a point along its length corresponding to apressure minimum or node of the standing wave to be suppressed. Byproper location of the fiow juncture between such inlet and capacitance,an entire family of standing waves can be suppressed. As another facetof this invention, the transmission of wave energy from a standing wavein an acoustical capacitance to an outlet conduit, such as an acousticalinductance, is minimized by positioning the juncture therebetween at apressure node of the standing wave. Here again, proper location of suchjuncture along the length of the capacitance can minimize thetransmission of sound energy from an entire family of standing waves. Aswill be shown hereinafter, certain combinations of such locations caneliminate resonant peaks from the filter frequency response curve formany of the frequencies with which the filter elements are in resonanceand which would otherwise be transmitted by the filter. Before turningto the details of the arrangements above generally described, it shouldbe pointed out that a preferred form of filter constructed in accordance.with this invention will have (1) the resonant frequencies of itscomponents separated as far as possible from the predominant(fundamental and lower harmonics thereof) frequencies of the pumpingdevice or compressor, (2) standing waves in the acoustical capacitancessuppressed by proper location of inlets thereto and (3) transmission ofwave energy from one filter component to another controlled by properlocation of junctures therebetween,

In discussing each of these three structural arrangements in detail,each will be discussed separately in the above order with appropriatetheory being given.

Every junction between pipes of difierent diameters acts as a reflectionpoint for an acoustic pressure wave traveling through the pipes. Anexample of such junction is that between an acoustic choke or inductanceand an acoustic capacitance in a filter, these elements commonly beingconstructed of pipe. It has been found that the resonant frequencies ofthe larger of the pipes (capacitance or bottle) are those of an excitedpipe essentially closed at both ends while the resonant frequencies ofthe smaller of the two pipes (choke) are those of an open ended organpipe. The large pipe or bottle tends to establish standing waves whichcreate a pressure maximum at the ends of the bottle while the smalldiameter pipe or choke tends to establish standing waves having apressure minimum or node at the ends of the pipe. In either case, thefrequencies at which the pipes are resonant to produce the standingwaves therein are related to the length of the particular pipe byF-=NV/2L, where P is a resonant frequency, N is an integer, V is thevelocity of sound in the fiuid and L is the acoustic length of the pipe.

This relationship holds in those instances where (l) the pipe diameteris small compared with the wave length of the sound in the pipe (lessthan approximately & of the wave length of the sonic vibration inquestion) and (2) the pipe length is of the same order of magnitude orlonger than the wave length of the sound. Thus, it can be seen thatstanding waves are established in gas-filled pipes (such as thosecomprising acoustic filter elements) at a frequency dependent upon thelength of the pipe.

In accordance with the preferred form of this invention, the lengths ofthe various components of the acoustic filter including associatedpiping are chosen so that the resonant frequencies of the components andof any in-line combination of the components are separated as far aspossible from the dominant frequencies (e.g. fundamental and lowerharmonics thereof, as well as subharmonics where existent) generated bythe compressor. In so doing, the fundamental pulsation frequency of thecompressor is first determined or calculated as being equal to thenumber of pulses of gas taken from the suction line and discharged intothe discharge line by the compressor per second. For example, acompressor consisting of 11 double acting cylinders, which reach the topdead center or some other common reference point at equal intervals ofcrank shaft angular position and which have dimensions of piston andcylinder such that an identical mass of gas is moved by each of the headand crank ends, would deliver 2n equally spaced pulses per revolution ofthe crank shaft. Therefore, if the crank shaft makes m revolutions persecond, the fundamental pulsation frequency delivered to the. suctionand discharge piping is 2nm pulses per second. This fundamentalfrequency and the first few harmonics thereof are those whichpredominate in the frequency spectrum of the pressure pulses deliveredto an acoustic filter in the suction or discharge piping of thecompressor. The lengths of the various components of the filter at whichthe components will be in resonance with the fundamental and first fewharmonics (e.g. first to fourth) can be determined from the formula L2FN where L is the acoustic length of a component, N is an integerchosen to be 1 for the fundamental frequency, 2 for the first harmonic,3 for the second harmonic, etc., V is the velocity of sound in the gas,and P is the fundamental or harmonic frequency of the compressor. Anyone component can be in resonance at any or all of the frequenciescorresponding to N=l, 2, 3, 4, and standing waves can exist in thecomponent for all such frequencies. The amplitude of each standing waveis dependent upon the frequency and amplitude of the pressure pulseswhich excite the component. Thus, if the component is excited by apressure wave of a frequency equal to or near one of its resonantfrequencies with an amplitude equal to or greater than that necessary tosus- D tain a standing wave at such frequency, very large amplitudestanding waves are established. To avoid this, the various filtercomponents, insofar as possible, are chosen to have a length such thatthey will be out of resonance with the fundamental and first fewharmonics thereof.

Of course, under actual operating conditions, the compressor speed m canbe expected to vary through a range of speeds thereby yielding bands offundamental and harmonic frequencies which must be considered. Since theband width increases with the order'of the harmonics, it may not, inmany instances, be possible to choose lengths of filter components outof resonance with all of the lower order harmonics but, in accordancewith the above, the acoustic length of the filter components, insofar aspossible, are chosen to be out of resonance with as many frequencies aspossible.

Of course, the components of the filter, that is, the acousticcapacitances and inductances, are sized as to volume, area ratios, etc.,to control the value of the low cut-off frequency of the filter and thenthe length of the components selected in accordance with the above. Themethod of sizing such components to obtain a particular value'ofcut-olffrequency is well known to those skilled in the art and need not beconsidered further here.

Now referring to Fig. 1, there is shown a compressor having suction anddischarge nozzles 11 and 12. Connected with these nozzles (which canthemselves be sized to act as acoustical inductances) are suction anddischarge filters of the series type. While there are several possiblearrangements of acoustical capacitances and inductances to form afilter, there is shown a series of acoustical capacitances in the formof bottles or chambers 13, 14 and 15 on the discharge side of thecompressor and bottles or chambers 16, 17 and 18 on the suction side.Interconnecting these bottles are acoustical inductances or chokes inthe form of conduits 19, '20 and-21 on the discharge side and conduits22, 23 and 24 on the suction side. Each of these conduits is of arelatively smaller cross-sectional area than that of the bottles.Conduit 21 connects with pipe line 25 into which the compressordischarges and conduit 24 connects with pipe line 26 from which suctionis taken and each of conduits 21 and 24 can be treated as inductances.cal capacitances (bottles) and acoustical inductances (chokes) are sizedso that the bottle volume, choke length, area ratios of the chokes andbottles, etc., are utilized to determine the value of the low cut-offfrequency of the entire filter on the suction and discharge sides of thecompressor. Also as indicated above, the fundamental frequency ofcompressor 10 is determined and then the lengths of the elements 11through 24 are made to be such that, insofar as possible, they willindividually not be in resonance with any one of the fundamentalfrequencies and low harmonics thereof generated by the compressor overits range of speeds. It is to be recognized, however, that because arange of frequencies are involved due to variations in compressor speedand because the range of resonant pipe frequencies are involved due tovariations in the speed of sound in the gas or fluid, it may not bepossible to provide lengths of all the components such as to be out ofresonance with all frequencies involved by a satisfactory amount.However, the lengths of the filter and piping elements are made suchthat the elements are not in resonance with as many of the frequenciesas is possible and practical.

In accordance with another aspect of this invention, the frequenciesgenerated by the compressor which tend to establish standing waves inthe acoustical capacitances or bottles are suppressed by connecting theflow inlet to the bottle to have a flow juncture therewith at a pressureminimum or node of the standing wave tending to be established in thebottle. In general, this flow juncture is 10- cated As indicated above,the various acousti distance from one end of the bottle, where L is theacous tic length of the bottle and n is an integer (l, 2, 3, 4, 5 Thus,as stated above, the bottles are in resonance at frequencies (F asdetermined by the formula Since the bottle acts as an excited pipeessentially closed at both ends, the standing wave pattern must be suchthat pressure maxima occur at the ends of the bottle. For all odd valuesof N, the standing wave pattern must be such that a pressure minimum, ornode, occurs at the center of the bottle For one-half of the even valuesof N (for N=2P where P is an odd-integer), the standing wave patternmust be such that a pressure minimum or node occurs at distances fromeach end of the bottle.

In accordance with this invention, the establishment of these standingwaves is suppressed by connecting the nozzles or flow inlets to theexpansion bottles at one of the distances of from the end of bottle 28and therefore standing waves having N values of 2, 6, l0, l4, l8, aresuppressed by interference. By situating inlet conduit 27 to have ajuncture at the mid point of bottle 28, standing waves which correspondto odd values of N would be suppressed. Similarly, by making the inletjuncture at distance from the end of the bottle, standing waves having Nvalues of 4, 12, 20,

As will be evident from Fig. 1, a plurality of bottles can have theirinlets 12, 19 and 20 respectively situated for different distances fromthe ends of the respective bottles according to the formula such as %andthereby suppressing by interference standing waves having odd values ofN, as well as those having even values of 2, 4, 6, l0, l2, 14, 18,20,

The same type of juncture location for suppression of standing waves canbe employed in the suction filter as.

indicated in Fig. 1 for the junctures of conduits 24, 23,

and 22 with bottles 18, 17 and 16, respectively.

As pointed out above, this invention affords a filter in which thetransmission of resonant frequencies between the filter and pipingelements is minimized through suitable location of the flow junctures ofthe outlets from the various acoustical capacitances or bottles. It hasbeen established that the pressure in the medium transmitting sonicvibrations must be continuous across any plane within the medium.Therefore, the pressure across the plane taken at that cross-section ofthe filter elements at which would likewise be suppressed.

7 those of different diameter form a junction, must be continuous. If anacoustical inductance or small diameter choke forming an outlet from alarger diameter capacitance or bottle is situated to have a flowjuncture with such bottle at the mid point of its length, it is joinedto a point of minimum pressure amplitude for all standing waves in thebottle having frequencies corresponding to oddvaluesof N in the formulaSince the pressure must be continuous across a plane lying at thejuncture, the coupling of pressure pulsations at such frequencies intothe choke is minimized and therefore the transmission of the wave energyof such pulsations to the next bottle or other piping is likewiseminimized. Such an arrangement is demonstrated in Fig. 3 where a bottle35 is shown with a fiow inlet 36 and an outlet 37 which can be a choke.A standing wave 38 having an N value of 7 is shown in the bottle andanother standing wave 39 in choke 37. The latter is connected to form aflow juncture at the mid point of the length of bottle 35 so that theplane 49 of the juncture intersects a pressure node 41 of wave 38.Accordingly, transmission of sound energy by wave 38 into choke 37 isminimized and theoretically can be made to be zero. Accordingly,transmission of sound energy by all standing waves having an N valueequal to an odd integer is minimized by the arrangement of Fig. 3.Similarly, by moving choke 37 to have its juncture with bottle 35 at adistance from one end of the bottle equal to (where L is the length ofthe bottle), transmission of sound energy by standing Waves having Nvalues of 2, 6, 10, 14 is minimized.

Therefore, in accordance with one aspect of this invention, the flowoutlets from the acoustic capacitances are connected as to have flowjunctures therewith at distances from one end of the capacitances, whereL is the length of the acoustical capacitances and n is an integer.Where a multiplicity of capacitances are employed in a filter, theoutlet flow junctures with the various capacitances can be situated sothat each is different from the others in its distance from the end ofits capacitance by making each juncture at a location corresponding to adifferent value of n. Thus, referring to Fig. 1 again, choke 19 isconnected to bottle 13 at distance from the end of the bottle. As aresult, transmission of sound energy from standing waves having oddvalues of N will be minimized. Similarly, choke 2.0 is connected to thebottle 14 at a distance from the end so that transmission of soundenergy from standing waves having even N values corrmponding to theseries 2, 6, l0, l4, is likewise minimized. Still further, choke 21 isconnected to capacitance 15 at a distance and 11 are respectivelyconnected to capacitances 18, 17 and 16 at distances equal to from theends of the respective capacitances.

With the foregoing arrangement, only a very few frequencies generated bythe compressor can be transmitted through the filter.

In the most preferred filter arrangement, the acoustic lengths of thevarious capacitances and inductances are made equal to each other or aremultiples of some basic length so that only a single resonance frequencyneed be dealt with. Then by arranging the flow junctures as abovetaught, the transmission of resonance peaks through the filter issatisfactorily prevented.

Thus, in a low band pass acoustic filter, there is usually employed twobottles interconnected by a choke. It is usually prefered to situate theoutlet flow junctures of each of the bottles at the mid point of thebottles length and each of the inlet flow junctures at one-fourth thelength of the bottles from the bottles ends. In such an arrangement, themid point connections tend to prevent transmission of Wave energy fromstanding waves having odd modes of vibration and the one-quarterconnections tend to suppress one-half the even modes of vibrationincluding the important second harmonic. Since there are, in effect, twomid point junctures in series, very little, if any, wave energy fromstanding waves of odd modes of vibration can be transmitted through thefilter. Thus, even though the amplitude of the standing waves in thefirst bottle is quite large and even though the mid point outletjuncture with such bottle is not completely effective so that someenergy is transmitted to the second bottle, the amplitude of thestanding waves thereby excited in the second bottle will be relativelysmall. Accordingly, the outlet flow juncture with the second bottle,even though it too is not completely effective, will still furtherreduce the amount of energy transferred to the piping.

The same is true of the inlet flow junctures because even though theinlet juncture with the first bottle does not entirely prevent theestablishment of standing waves therein corresponding to N equals 2, 6,10, etc., the inlet juncture with the second bottle still furthersuppresses their formation in the second bottle so that the resultantenergy transferred to the piping is insignificant.

On the other hand, if several different acoustic lengths of chokes andbottles are chosen for the several components of the filter, severalresonance peaks will tend to be present in the frequency response curvefor the filter. Any one of these peaks (and harmonics of thecorresponding frequency) can be reduced by proper junction location.However, from a practical standpoint, it will be necessary to provide atleast one set of mid point and quarter point connections for eachdifferent length of choke or bottle to take care of the resonantfrequencies of such lengths. This may be physically impossible (i.e.where the lengths are all different) or may require an excessive numberof chokes and bottles.

The fact that a single juncture, either inlet or outlet, may not becompletely effective for its intended purpose as above indicated, i.e.it may not result in complete sup-- pression of a standing wave or itmay permit some energy to be transmitted from a bottle, may be becausethe juncture is not a true plane lying exactly at a node of a standingwave, i.e. the tuning is not sharp. This may arise from: (1) the gaseousmedium is moving through the bottle so that the speed of wave travelthrough the bottle is greater in one direction than in an oppositedirection; (2) the ends of the bottle frequently are not planar but theends may be cup or dish shaped thereby causing slurring of the resonantfrequency of the bottle; (3) the choke may not terminate in a planenormal to the longiutdinal axis of the bottle but may extend into I hithe bottle from the side thereof so that the end of the choke lies in aplane parallel to the axis of the bottle or has some finite width alongthe length of the bottle whereby the choke-bottle flow juncture extendsto either side of a node. Accordingly, from a practical standpoint, itis often preferred to use two mid point and quarter point junctioncombinations in series witheach other to effect maximum filterefliciency. This can be most simply accomplished when the filterelements are all of equal acoustic lengths since only a minimum numberof junctures are then required. I

It should alsobe pointed out that when a choke is made of equal acousticlength to that of a bottle from which it conducts flowing gas, anytransfer of energy from a standing wave in the bottle to the choke tendsto setup a standing wave in the choke. Such a standing wave has apressure node at the outlet end of the choke so that only a minimum, ifany, energy from the standing wave in the choke is transferred out ofits outlet end. In other words, the energy is reflected from the outletend back towards the inlet end. As a result, a very little energy at theresonant frequency of the choke can be transferred to succeeding piping.However, here again, the choke may not terminate at its outlet endexactly at a node so that the reflection may not be complete.

It is also possible by this invention to select a bottle or choke whichis to be subjected to the maximum amount of mechanical vibration arisingfrom the filtering action while vibration in other bottles or chokes isminimized. Thus, if the second bottle away from the compressor is to besubjected to the maximum vibration in dampening standing waves,- thejunctures to it will be connected at its mid and quarter points, forexample, while the junctures with the other bottles are connected atpoints other thanthose corresponding to a pressure node. In other words,the junctures with the other bottles are such that standing waves arenot suppressed therein.

Referring now to Fig. 4, there is shown a band elimination filter(resonator) comprising a bottle 42 connected as a side branch (in shunt)to a compressor discharge line 43. The discharge line is connected to apipe line 44 or to other elements of an acoustic filter. In thisarrangement, bottle 42 can be connected through a choke 45 to the flowline so that steady flow from the compressor does not pass through thebottle or choke but instead, the bottle and choke are subject only topulsing flow. Such a filter is designed to temporarily store pulsationsof a given frequency and to reintroduce these pulsations into the flowline to which it is coupled 180 out-of-phase with the originalpulsations. This out-of-phase introduction of pulsations has the elfectof smoothing out the original pulsations in the flow line.

For the resonator to have maximum effect at a particular frequency, itmust be coupled to the steady flow piping at a point where the pressurevariation for that particular frequency is greatest. By connecting at apoint of peak pressure variation (e.g. at an antinode), an optimumamount of pulsating energy at the frequency to be eliminated isalternately stored and released by the side branch filter into thesystem. Thus, if the steady flow system comprising piping 43 is resonantto a particular frequency in the output of compressor 10, the sidebranch filter is connected at a point along the length of pipe 43corresponding to an antinode of the standing wave therein. The acousticlength of the sidebranch filter is such that pulses are reintroduced 180out-ofphase with the original pulsations.

Reference has been made herein to acoustic length. This term is meant toinclude the actual length of the choke, bottle or pipe plus a length tocorrect for the end effect of the choke, bottle or pipe. While theacoustic length is preferably employed as L in the above formulas, itmay be possible in some instances to use the actual length and yetrealize the advantages of this invention to a lesser degree.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

. The invention having been described, what is claimed 1. Apparatus fordampening pulsations in a fluid stream having pulsating flow created bya fluid pumping device generating an acoustic pressure wave of complexharmonic structure and for minimizing transmission of pulsating soundenergy from the device which comprises: means including acousticalcapacitance and acoustical in ductance units having connections witheach other to form a fluid pulsation dampening filter and also havingconnections to apply the pulsating flow of said fluid stream to thefilter, said acoustical capacitance unit being of an acoustic length Las to be resonant with at least one of the fundamental and harmonicfrequencies of said pressure wave to thereby cause a standing wave insaid capacitance unit at such one frequency, one connection with saidcapacitance unit forming a juncture therewith at a location which isdistance from one end of the capacitance unit, n being an integer.

2. Apparatus for dampening pulsations in a fluid stream having pulsatingflow created by a fluid pumping device generating an acoustic pressurewave of complex harmonic structure and for minimizing transmission ofpulsating sound energy from the device which comprises: a pair ofacoustical capacitance units with at least one of the units having alength as to be resonant with at least one of the fundamental andharmonic frequencies comprising said pressure wave to thereby form astanding wave in such unit at such frequency, means forming anacoustical inductance gas passage means interconnecting said capacitanceunits, said acoustical inductance means having a flow juncture with saidunit having said standing wave therein at a location 2" feet from oneend of such unit, where L is the acoustic length of the unit in feet andn is an integer.

3. The apparatus of claim 2 wherein said juncture is at the mid point ofthe acoustic length of the unit to thereby minimize transmission to saidacoustical inductance means of pulsating sound energy from standingwaves having a pressure node at such mid point.

4. The apparatus of claim 3 in combination with gas passage meansconnected to the other of said units to provide a flow juncturetherewith at a location feet from one end of such other unit, where Listhe acoustic length of such other unit in feet and n is an integerhaving a value greater than unity to thereby suppress transmission fromsuch other unit of at least some of the standing waves therein havingeven modes of vibration.

5. Apparatus for dampening pulsations in a fluid stream having pulsatingflow created by a fluid pumping device generating an acoustic pressurewave of complex:

harmonic structure and for minimizing transmission of pulsating soundenergy from the device which comprises: a pair of acoustical capacitanceunits and acoustical inductance units connecting between said acousticalcapacitance units for fluid flow therebetween and with the acousticalcapacitance units forming a pulsation dampening filter, a how connectionto the filter, one of said acoustical capacitance units being of alength as to be resonant with at least one of the fundamental andharmonic frequencies comprising the pressure wave thereby tending toform a standing wave in said one acoustical capacitance unit, saidacoustical inductance unit having a flow juncture with said oneacoustical capacitance unit at a location therein at which a pressurenode of said standing wave occurs thereby minimizing transmission ofpulsating sound energy to said acoustical inductance unit at a frequencycorresponding to that of said standing wave.

6. The apparatus of claim wherein said one acoustical capacitance unithas a relatively smaller flow connection for discharge of fluid intosaid one unit, said flow connection having a flow juncture With said oneunit at a location corresponding to a pressure node of the standing wavein said one unit thereby dampening by interference said standing wave.

7. The apparatus of claim 6 wherein the other of said acousticalcapacitance units is also of a length as to be resonant with at leastone of the fundamental and harmonic frequencies of the pressure wavethereby permitting a standing wave to be formed therein, and a flowconnection with said other acoustical capacitance unit having a flowjuncture therewith at a location corresponding to that of a pressurenode of said standing wave in said other acoustical capacitance unit.

8. The apparatus of claim 7 wherein said acoustical inductance unit hasa flow juncture with said other acoustical capacitance unit at alocation therein at which a pressure node of said standing wave occursin said other unit thereby dampening by interference said standing wavetherein.

9. The apparatus of claim 5 wherein said acoustical capacitance unitsand said acoustical inductance units are related as to the acousticlength of each by the following formula:

Lin is equal to L n is equal to L371.

wherein L L and L are the respective acoustic lengths of the units, andn, n and n" are integers, whereby transmission through the filter ofpressure wave frequencies with which the units are resonant issuppressed.

10. The apparatus of claim 9 wherein n, n, and n are equal to eachother.

11. Apparatus for dampening pulsations in a fluid stream havingpulsating flow created by a fluid pumping device generating an acousticpressure wave of complex harmonic structure and for minimizingtransmission of pulsating sound energy from the device which comprises:a pulsation dampening filter including a pair of relatively largeacoustical capacitance chambers interconnected by a relatively smallconduit forming an acoustical inductance between the chambers and havingflow junctures with each chamber, said chambers and inductance conduiteach having a length as to be out of resonance with at least saidfundamental frequency and in resonance with a higher harmonic frequencythereby tending to form a standirn wave therein at such higherfrequency, a fiow connection having a flow juncture with one of saidchambers, at least one of the flow junctures with said chambers being ata distance from one end thereof of where L is the acoustic length ofsuch chamber and n is an integer.

12. The apparatus of claim 11 wherein said flow connection juncture isat a distance of from one end of said one chamber and said acousticalinductance conduit juncture with the other chamber is at distance fromone end of said other chamber, where L and L are the acoustic lengths ofsaid one and other chambers, respectively, and are equal, and where n isl in one instance and an integer larger than 1 in the other instance.

13. The apparatus of claim 12 where n is 2 in said other instance.

14. The apparatus of claim 12 wherein the juncture of said inductanceconduit with said one chamber is at a location corresponding to apressure node of a standing wave therein.

15. The apparatus of claim 11 which includes a flow connection with theother of said chambers and having a flow juncture therewith at adistance of acoustical inductance conduit juncture with said one chamberis at distance from one end of said one chamber, where L; and L are theacoustic lengths of said one and other chambers, respectively, and areequal, and Where n is 1 in one instance and an integer larger than 1 inthe other instance.

16. The apparatus of claim 15 wherein n is 2 in said other instance.

17. The apparatus of claim 15 wherein the juncture of said acousticalinductance conduit with said other chamber is at a locationcorresponding to a pressure node of the standing wave therein.

18. Apparatus for dampening pulsations in a fluid stream havingpulsating flow created by a fluid pumping device generating an acousticpressure wave of complex harmonic structure and for minimizingtransmission of pulsating sound energy from the device which comprises:a flow line for said stream, an acoustical capacitance coupled to saidflow line as a side branch and thereby subjected to pulsating flow only,the capacitance being coupled to said flow line at an antinode of astanding wave tending to exist in the flow line, said side branch havingan acoustic length such that pulsating waves of pressure passingthereinto from the flow line are reflected back to the flow lineout-of-phase with said standing wave.

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